Add last_value to ScalarTimeSeries interface. (#6579)

This allows `list_scalars` of some of the data provider implementations to return the last scalar value, which will improve the performance when loading experiments with Hparams data.

Googlers, see b/292102513 for context.

Tested internally: cl/563163418

#hparams

Author: yatbear

tensorboard/data/provider.py

@@ -777,6 +777,7 @@ class _TimeSeries:
         "_plugin_content",
         "_description",
         "_display_name",
+        "_last_value",
     )
 
     def __init__(
@@ -787,12 +788,14 @@ def __init__(
         plugin_content,
         description,
         display_name,
+        last_value=None,
     ):
         self._max_step = max_step
         self._max_wall_time = max_wall_time
         self._plugin_content = plugin_content
         self._description = description
         self._display_name = display_name
+        self._last_value = last_value
 
     @property
     def max_step(self):
@@ -814,6 +817,10 @@ def description(self):
     def display_name(self):
         return self._display_name
 
+    @property
+    def last_value(self):
+        return self._last_value
+
 
 class ScalarTimeSeries(_TimeSeries):
     """Metadata about a scalar time series for a particular run and tag.
@@ -830,6 +837,9 @@ class ScalarTimeSeries(_TimeSeries):
         empty if no description was specified.
       display_name: An optional long-form Markdown description, as a `str` that is
         empty if no description was specified. Deprecated; may be removed soon.
+      last_value: An optional value for the latest scalar in the time series,
+        corresponding to the scalar at `max_step`. Note that this field might NOT
+        be populated by all data provider implementations.
     """
 
     def __eq__(self, other):
@@ -845,6 +855,8 @@ def __eq__(self, other):
             return False
         if self._display_name != other._display_name:
             return False
+        if self._last_value != other._last_value:
+            return False
         return True
 
     def __hash__(self):
@@ -855,6 +867,7 @@ def __hash__(self):
                 self._plugin_content,
                 self._description,
                 self._display_name,
+                self._last_value,
             )
         )
 
@@ -866,6 +879,7 @@ def __repr__(self):
                 "plugin_content=%r" % (self._plugin_content,),
                 "description=%r" % (self._description,),
                 "display_name=%r" % (self._display_name,),
+                "last_value=%r" % (self._last_value,),
             )
         )
 

tensorboard/data/provider_test.py

@@ -91,7 +91,13 @@ def test_repr(self):
 
 class ScalarTimeSeriesTest(tb_test.TestCase):
     def _scalar_time_series(
-        self, max_step, max_wall_time, plugin_content, description, display_name
+        self,
+        max_step,
+        max_wall_time,
+        plugin_content,
+        description,
+        display_name,
+        last_value,
     ):
         # Helper to use explicit kwargs.
         return provider.ScalarTimeSeries(
@@ -100,6 +106,7 @@ def _scalar_time_series(
             plugin_content=plugin_content,
             description=description,
             display_name=display_name,
+            last_value=last_value,
         )
 
     def test_repr(self):
@@ -109,26 +116,28 @@ def test_repr(self):
             plugin_content=b"AB\xCD\xEF!\x00",
             description="test test",
             display_name="one two",
+            last_value=0.0001,
         )
         repr_ = repr(x)
         self.assertIn(repr(x.max_step), repr_)
         self.assertIn(repr(x.max_wall_time), repr_)
         self.assertIn(repr(x.plugin_content), repr_)
         self.assertIn(repr(x.description), repr_)
         self.assertIn(repr(x.display_name), repr_)
+        self.assertIn(repr(x.last_value), repr_)
 
     def test_eq(self):
-        x1 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two")
-        x2 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two")
-        x3 = self._scalar_time_series(66, 4321.0, b"\x7F", "hmm", "hum")
+        x1 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two", 512)
+        x2 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two", 512)
+        x3 = self._scalar_time_series(66, 4321.0, b"\x7F", "hmm", "hum", 1024)
         self.assertEqual(x1, x2)
         self.assertNotEqual(x1, x3)
         self.assertNotEqual(x1, object())
 
     def test_hash(self):
-        x1 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two")
-        x2 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two")
-        x3 = self._scalar_time_series(66, 4321.0, b"\x7F", "hmm", "hum")
+        x1 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two", 512)
+        x2 = self._scalar_time_series(77, 1234.5, b"\x12", "one", "two", 512)
+        x3 = self._scalar_time_series(66, 4321.0, b"\x7F", "hmm", "hum", 1024)
         self.assertEqual(hash(x1), hash(x2))
         # The next check is technically not required by the `__hash__`
         # contract, but _should_ pass; failure on this assertion would at